A conventional shadow-mask-type CRT comprises an evacuated envelope having therein a viewing screen comprising an array of phosphor elements of three different emission colors arranged in a cyclic order, means for producing three convergent electron beams directed towards the screen, and a color selection structure or shadow mask comprising a thin multi-apertured sheet of metal precisely disposed between the screen and the beam-producing means. The apertured metal sheet shadows the screen, and the differences in incidence angles permit the transmitted portions of each beam to selectively excite only phosphor elements of the desired emission color. A matrix of light-absorptive material surrounds the phosphor elements.
U.S. Pat. No. 3,475,169 issued to H. G. Lange on Oct. 28, 1969 discloses a process for electrophotographically screening color cathode-ray tubes. The inner surface of the faceplate of the CRT is coated with a volatilizable conductive material and then overcoated with a layer of volatilizable photoconductive material. The photoconductive layer is then uniformly charged, selectively exposed with light through the shadow mask to establish a latent charge image, and developed using a high molecular weight carrier liquid bearing, in suspension, a quantity of phosphor particles of a given emissive color that are selectively deposited onto suitably charged areas of the photoconductive layer. The charging, exposing and deposition processes are repeated for each of the three color-emissive phosphors, i.e., green, blue, and red, phosphors of the screen.
An improvement in electrophotographic screening is described in U.S. Pat. No. 4,921,767, issued to P. Datta et al. on May 1, 1990, wherein the method thereof uses dry-powdered, triboelectrically-charged screen structure materials having at least a surface charge control agent thereon to control the triboelectrical charging of the materials. Such a method decreases manufacturing time and cost, because fewer steps are required for "dry-processing" of both the matrix and phosphor materials. A drawback of the described method is that some cross-contamination or background deposition may occur, because of electrostatic field variations near the photoconductor which do not effectively repel all the positively charged phosphor particles from selected regions of the photoconductor as described below.
Accordingly, a need exists for a means of electrophotographically manufacturing screen assemblies using dry-powdered, triboelectrically-charged phosphor materials, without cross-contamination of the different color-emitting materials.